The Star-Forming Galaxy Contribution to the Cosmic MeV and GeV Gamma-Ray Background

TL;DR

This paper models the gamma-ray emission from star-forming galaxies across MeV to GeV energies, assessing their contribution to the cosmic gamma-ray background and exploring emission mechanisms and uncertainties.

Contribution

It introduces comprehensive one-zone models including multiple emission processes to estimate star-forming galaxies' gamma-ray background contribution, especially in the MeV range.

Findings

Star-forming galaxies likely contribute most of the unresolved GeV background.

Starburst galaxies may emit significant MeV gamma rays if magnetic fields are low.

The contribution of star-forming galaxies to the 1 MeV background is limited to less than 10%.

Abstract

While star-forming galaxies could be major contributors to the cosmic GeV $\gamma$-ray background, they are expected to be MeV-dim because of the "pion bump" falling off below ~100 MeV. However, there are very few observations of galaxies in the MeV range, and other emission processes could be present. We investigate the MeV background from star-forming galaxies by running one-zone models of cosmic ray populations, including Inverse Compton and bremsstrahlung, as well as nuclear lines (including $^{26}$Al), emission from core-collapse supernovae, and positron annihilation emission, in addition to the pionic emission. We use the Milky Way and M82 as templates of normal and starburst galaxies, and compare our models to radio and GeV--TeV $\gamma$-ray data. We find that (1) higher gas densities in high-z normal galaxies lead to a strong pion bump, (2) starbursts may have significant MeV emission if their magnetic field strengths are low, and (3) cascades can contribute to the MeV emission of starbursts if they emit mainly hadronic $\gamma$-rays. Our fiducial model predicts that most of the unresolved GeV background is from star-forming galaxies, but this prediction is uncertain by an order of magnitude. About ~2% of the claimed 1 MeV background is diffuse emission from star-forming galaxies; we place a firm upper limit of <~10% based on the spectral shape of the background. The star-formation contribution is constrained to be small, because its spectrum is peaked, while the observed background is steeply falling with energy through the MeV-GeV range.